I just got Monster Rancher Battle Card Episode II, and I love it. It's the best card game I played since Culdcept, and it doesn't have the merciless computer players. The only thing I hate is the fact that I spent too much time letting the PSone read CDs instead of actually battling.
I haven't tried the other Monster Rancher games, yet but I'm planning on it now. I just noticed that my PSone will allow me to use CD-Rs to create monsters, so I wonder how that will affect me getting rare cards.
Still, I don't know why anyone would want to play Monster Rancher on a system like the Game Boy Advance if you can't read the CDs or DVDs on it.